The recently increased demand for portable electronic products such as laptop computers, video cameras, mobile phones, and so on, and the development of energy storage batteries, robots, satellites, and so on started in earnest led into active researches on high-performance secondary batteries capable of repeated charging and discharging.
Currently, commercially available secondary batteries comprise nickel cadmium, nickel hydrogen, nickel zinc and lithium secondary batteries and so on. Among them, lithium secondary batteries have drawn much attention because of their advantages of little memory effect to allow unrestrained charging/discharging, as well as very low self-discharging rate and high energy density and so on, compared to nickel-based secondary batteries.
However, such secondary batteries naturally degrade over time or degrade due to repeated charging/discharging, thereby deteriorating their performance. Therefore, in using a secondary battery, a technique of quantitatively evaluating a service life of the secondary battery is required.
The remaining service life of a battery is a parameter that quantitatively represents a change in capacity characteristics of the battery, which enables a battery user to know about how much capacity of the battery remains in the battery. Such an SOH generally represents capacity of the battery in percentage (%). For example, the SOH being 97% means that the remaining service life of a battery is 97% compared to its initial state, in other words, a state where 3% of capacity degradation occurred.
Therefore, using the SOH, a battery user may replace the battery at an appropriate time point, and adjust the charge/discharge capacity of the battery according to the period of use of the battery, thereby preventing over-charge and over-discharge of the battery.
Meanwhile, there are a wide variety of methods for estimating such an SOH, for example, there exists a method of estimating the SOH using an internal resistance and temperature of the battery, and a method of estimating the SOH through a full-discharge test, etc. However, such a conventional SOH estimating method has a problem that there has to be a change in parameter of the battery (mainly, a change in an electrical parameter) to enable estimation of the SHO. That is, when the battery is charged/discharged, a change in parameter of the battery occurs sufficiently, but when the battery is not being used, a change in parameter of the battery does not occur sufficiently enough to enable estimation of the SOH. Therefore, when a state continues where the battery is not being used, the parameter of the battery does no change sufficiently enough to enable estimation of the SOH. This leads to a problem where SOH estimation of the battery is not performed, and thus a proper SOH cannot be provided to the user. That is, there occurs a problem where even when the battery is degraded as time elapses, the user is provided with not a changed SOH but a previously estimated SOH.
A particular problem is when the battery is not used for a long period of time. When the battery is not used for a long period of time, SOH estimation is not performed, and thus even if the battery is degraded sufficiently due to natural degradation, the user cannot be provided with the SOH where such natural degradation is reflected.
When the user uses the battery at a state where the battery has not been used for a long period of time as aforementioned, a change in parameter of the battery would occur at last at the time point the battery is used and SOH estimation would be performed, providing an SOH estimate value to the user. Here, since a degree of degradation caused by natural degradation is reflected, the user would recognize at last that the remaining service life of the battery has changed drastically.
As a result, the user may determine that the apparatus for estimating a remaining service life of a battery cannot be relied upon, or miscalculate that the battery has broken down. Due to the aforementioned, an unnecessary inspection may be made to determine whether the battery has broken down, and the battery may be replaced unnecessarily, and thus there is concern of wasting the costs.